1. Technical Field
The present invention relates to a magnetic sensor having a Spin-valve type giant magnetoresistive element and a magnetic encoder using the same.
2. Background Art
Recently, there have been strong demands that a magnetic encoder for use in consumer electronics equipment such as a digital still camera and an ink jet printer achieves high resolution and low power consumption in addition to small size and low price.
Heretofore, an anisotropic magnetoresitance effect (hereinafter referred to simply as “AMR”) film made of a NiFe (nickel-iron) alloy film or the like has been used for a magnetic sensor to be mounted to the magnetic encoder. The AMR effect is a phenomenon in which electrical resistance changes according to a relative angle between the direction of current passing through a ferromagnetic film of a NiFe alloy or the like and the direction of magnetization of the ferromagnetic film. By utilizing the phenomenon, a change in resistance of an element according to an externally applied signal magnetic field can be outputted through a change in voltage or current. Specifically, when an AMR element is disposed as separated by a predetermined gap from a magnetic medium magnetized in alternating multipolar form as illustrated schematically in FIG. 1, a change in output according to a periodic signal magnetic field originating from the magnetic medium can be detected.
High resolution of the magnetic encoder can be achieved by narrowing a magnetized pitch (or a length of a pair of the north and south poles) of the magnetic medium and correspondingly narrowing a pattern width of the magnetic sensor. However, it is required that the magnetic sensor be of high sensitivity because the narrowing of the magnetized pitch of the magnetic medium leads to a decrease in the signal magnetic field from the surface of the magnetic medium. Although an AMR film of NiFe or the like undergoes a change in electrical resistance under a signal magnetic field of relatively small magnitude, magnetoresistive ratio (hereinafter referred to simply as “MR ratio”) is of the order of a few percent, which is not necessarily high. Thus, the sensitivity can possibly be insufficient for the magnetic encoder to achieve high resolution. Generally, the AMR film has a thickness of about 20 nm. Thus, the narrowing of the pattern width of the magnetic sensor corresponding to the magnetized pitch of the magnetic medium leads to shape anisotropy, which can possibly cause an increase in an anisotropy field and hence a reduction in the sensitivity to magnetic field. Moreover, the thick AMR film means that the resistance of the element is relatively low, and therefore the AMR film has a problem also from the viewpoint of power consumption.
Other magnetic sensors include an element utilizing an antiferro-coupled giant magnetoresistive (hereinafter referred to simply as “coupled GMR”) film, as disclosed in Japanese Patent No. 2812042. The coupled GMR film is formed of a multilayer superlattice film having ferromagnetic layers and non-magnetic layers alternating with each other, which are stacked one on top of another in a few layers to a few tens of layers. Antiferromagnetic interlayer coupling (or interaction such that the magnetization directions of adjacent ferromagnetic layers are antiparallel to each other) occurs between the adjacent ferromagnetic layers with the non-magnetic layer in between. In the GMR film, an electrical resistance changes according to a relative angle between the magnetization directions of the adjacent ferromagnetic layers with the non-magnetic layer in between. More specifically, under no external magnetic field, the magnetizations of the adjacent ferromagnetic layers are antiparallel to each other, and the resistance is maximized. On the other hand, under an external magnetic field, the magnetizations of the adjacent ferromagnetic layers are parallel to each other, and the resistance is minimized. The MR ratio of the coupled GMR film is a few times higher than that of the AMR film, and therefore the coupled GMR film is advantageous in terms of high output. However, a transition of the magnetizations of the adjacent ferromagnetic layers from an antiparallel state to a parallel state requires a magnetic field of such great magnitude that overcomes the antiferromagnetic interlayer coupling between the ferromagnetic layers with the non-magnetic layer in between. The coupled GMR film, in an aspect, cannot be said to be suitable for use in the magnetic encoder for detection of a signal magnetic field of relatively small magnitude. Moreover, the coupled GMR film has difficulty in achieving low power consumption because of having a thick sensor film and hence a low element resistance, as in the case of the AMR film.
Japanese Patent No. 3040750 discloses a Spin-valve type GMR film in use as a magnetic read head for a hard disk drive, as a magnetic sensor film that responds to a signal magnetic field of relatively small magnitude and exhibits a high MR ratio which is about the same as that of the coupled GMR film. The Spin-valve type GMR film is configured basically of a ferromagnetic pinned layer, a non-magnetic intermediate layer, and a ferromagnetic free layer. The direction of magnetization of the ferromagnetic pinned layer is unidirectionally pinned by an antiferromagnetic layer, which is formed adjacent to the ferromagnetic pinned layer to impart unidirectional magnetic anisotropy to the ferromagnetic pinned layer and do the like. On the other hand, the ferromagnetic free layer changes the direction of magnetization according to an external magnetic field. Thus, the Spin-valve type GMR film enables the transition of the magnetizations of the two ferromagnetic layers with the non-magnetic intermediate layer in between from the antiparallel state to the parallel state, under a magnetic field of relatively small magnitude. Moreover, the Spin-valve type GMR film has electrical resistance a few times higher than that of the coupled GMR film, and therefore the Spin-valve type GMR film is advantageous also in terms of low power consumption. A bridge circuit magnetic sensor using a Spin-valve type GMR element is disclosed in Japanese Patent No. 3017061.